Trd temperature sensor

ABSTRACT

A temperature sensing system has a signal means which provides a signal representative of a temperature responsive luminescence, where the luminescence has a characteristic time-rate-of-decay. A means for comparison is connected to the signal means and samples the signal during two time intervals, the first interval overlapping the second. The averages of the samples are compared to provide a difference signal representative of the difference between the two measured averages. Control means coupled to the comparison means provide an output representing the temperature as a function of the time-rate-of-decay, by adjusting the overlapping intervals so that the difference signal converges to a preselected limit.

This application is a division of U.S. application Ser. No. 07/599,814filed Oct. 18, 1990 and assigned to the same assignee as the presentapplication.

BACKGROUND OF THE INVENTION

The present invention relates to optical temperature measurement using atime rate of decay measurement.

In passive optical temperature sensing, a time-rate-of-decay (TRD)temperature sensing probe is thermally coupled to an object to measureits temperature. The probe tip includes luminescent material emittingradiation at a wavelength characteristic to the material upon excitationby radiation at another material characteristic wavelength. Theluminescent material is typically in the form of a sectioned, polishedcrystal or a powder embedded in a binder. Both the emission and theexcitation wavelengths are determined by the type of luminescentmaterial in the probe, and the intensity of the excitation issignificantly more intense than that of the emitted radiation. Theluminescent emission decays substantially exponentially with time andthe exponential time constant of the emission curve is responsive to thetemperature.

Luminescence is characterized by light emission from matter anddescribes several processes resulting in such emission. The luminescentemission intensity, lifetime and frequency spectrum can be temperaturedependent, and therefore one or more of these parameters may be used asa thermoresponse parameter. Fluorescence and phosphorescence are twoemissive processes defined quantum mechanically. Quantum mechanics teachthat an electron surrounding an atomic nucleus has specific quantized,allowed states of energy characterized by quantum numbers. One of thequantum numbers is the spin quantum number, S, which is a measure of theangular momentum of the electron orbit in the energy state. Spin orbitquantum numbers are given by S=1/2±n, where n is an integer. When amagnetic field is present within the atom, a spin orbit state may splitinto two allowed states by a process called spin-orbit splitting. Theenergy difference between the split states is proportional to the atomicnumber. Consequently, materials with a large atomic number which alsohave atomic magnetic fields have multiple states between which electronsmay transition.

An atom phosphoresces when an electron makes a transition from one statehaving a spin orbit quantum number S to a second state with the samenumerical quantum number, S. Phosphorescence is characterized by arelatively long emissive duration, between a microsecond and 1×10³seconds. Fluorescence occurs when the electron transition occurs betweenstates of different numerical spin quantum number and is characterizedby emissions of comparatively short lifetime, from 1×10⁻² seconds to1×10⁻¹⁰ seconds. In most cases, activated interstitial atomicimpurities, typically called "dopant" atoms in the literature, providefree electrons. Fluorescence and phosphorescence may occur in the samematerial if there are a substantial number of free electrons and spinorbit coupling, since the electrons are provided with multiple energystates of differing S.

The literature defining luminescent, fluorescent and phosphorescentprocesses is inconsistent. Sometimes processes are identified bymeasurement of the duration of the emission; sometimes, they areidentified by a quantum mechanical analysis of the problem. In thispatent application, the terms fluorescence and phosphorescence aredefined as discussed above. Additionally, luminescent materials withactivation sites having a high atomic number, which may luminesce viaphosphorescence, fluorescence or both processes will be exclusivelycalled luminescent materials. Readers are referred to Kittel,Introduction to Solid State Physics, Wiley, 1976 for further reading.

In a typical TRD probe, luminescent material is located in the probetip. Upon excitation, a luminescent emission occurs which is coupled toa detector by an optical fiber. The detector converts the emission intoa current having an amplitude varying with the emissive intensity.Electronics process the detector output by various means to determinethe substantially exponential time constant. The decay characteristicsof the detector output is substantially exponential but may becharacterized linearly or having other functionality over variousintervals of time. Once the time constant is measured, a look-up tableor an equation curve-fitted to empirical data is used to calculatetemperature.

The form of the luminescent material in the probe has some limitations,however. Some probes have a solid piece of luminescent material in theprobe tip which may be crystalline or amorphous. In this application thepiece of luminescent material will be called a crystal. Such luminescentprobe tip materials are called crystals in this application. Otherprobes have a solid piece of luminescent material made of powderedcrystalline or amorphous material embedded in a binder. Still otherprobes have a powder without a binder in the probe tip. Sectioning andpolishing can break crystals during manufacture. Furthermore, there is awide variation in luminescent signal intensities so that the electronicsare individually adjusted for each crystal probe tip. Such adjustment islabor intensive and time consuming in a manufacturing environment. Whilethe powdered form embedded in a binder improves the amount of intensityvariation between crystals compared to that of a polished crystal, theresulting emissive intensity is still orders of magnitude lower than theexcitation radiation. Therefore, the embedded powdered form results in amore easily interchangeable probe tip than does the polished crystal,but is still not optimized for a manufacturing process.

Sensitivity and intensity of luminescent emission over the temperaturerange of interest can be optimized by proper material choice. However,the emission intensity or low sensitivity of some materials is stilloften comparable to noise, requiring excessive electronic amplificationand subsequent distortion.

Various types of electronics are used to measure a quantity related tothe time rate of decay of the emission. One method measures the time fora preselected emissive intensity to halve. This method is mostsuccessful when the signal intensity is large compared to the backgroundnoise. Digital techniques digitize the emission at a high sampling rateand curve fit the resulting sampled emission curve. One analog techniquemeasures a phase difference between the excitation radiation and theemission and correlates the phase difference to the temperature. Somemethods of measuring time rate of decay are susceptible to backgroundlevel variations or drift, as well as variations in signal level.

Therefore, there is a need for a temperature probe having sufficientlystrong emission intensities to preclude excessive amplification, andsignal processing which is insensitive to background level and signallevel variations.

SUMMARY OF THE INVENTION

The present invention relates to a temperature measurement system formeasuring the thermally responsive time-rate-of-decay characteristic ofa luminescent signal.

The invention is practiced in a temperature sensing system, where thesystem has a signal means for providing a signal representative of atemperature responsive luminescence having a characteristictime-rate-of-decay and has comparison means coupled to the signal meansfor sampling the signal during two time intervals, the first intervaloverlapping the second. The comparison means compares the average valuesof the samples to provide a difference signal representative of thedifference therebetween. Control means coupled to the comparison meansprovide an output representing the temperature as a function of thetime-rate-of-decay, by adjusting the intervals so that the differencesignal converges to a preselected limit.

The comparison means preferably include autozeroing means whichsubstantially remove an undesirable DC offset from the luminescentsignal before the sampling.

In the present invention, the luminescent material in the signal meansis preferably in a powder form where a gas separates the particles inthe powder. The powder is made of a luminescent material containingchromium such as CR:GGG, Cr:GSGG and Cr:Beryl, and is pressed into atube by an optical fiber which conducts the luminescence to thecomparison and control means in the system. The end of the optical fiberwhich is pressed into the powder is preferably tapered. Alternatively,other means for increasing the numerical aperture of the fiber may beused such as an integral ball lens or a separate lens. The powdersubstantially comprises particles having a diameter less than two-thirdthe diameter of the optical fiber. The luminescent material can becoated with an inert material which is an efficient absorber foremitting blackbody radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a TRD sensing application including a TRD temperature probeand signal processing electronics;

FIG. 2 is a block diagram of the signal processing electronics of FIG.1;

FIG. 3 is a representation of the signals in the electronics shown inFIG. 2;

FIG. 4 is a flow chart showing operation of control circuit 50 in FIG.2;

FIG. 5 is a cross sectional drawing of TRD probe 14;

FIGS. 5A, 5B and 5C are cross sectional drawings of a TRD probe havingvarious fiber tips;

FIG. 6 is a cross sectional drawing of combination blackbody and TRDtemperature probe 300;

FIG. 7 is a drawing of a TRD temperature sensing application includingthree probes, three couplers and a common optical fiber highway; and

FIGS. 7A, 7B and 7C illustrate the spectral characteristics of the threecouplers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a temperature sensing system using a TRD temperature probe.Processing electronics 20 send signals to a source 4 over cabling 22which cause source 4 to emit light. Source 4 is preferably an LED butmay be an alternative source such as a laser. Some of the light iscoupled to an optical fiber 6, through optical coupler 8, through anoptical fiber 10 and to a connector 12. Alternately, optical fiber 6 maybe eliminated. Connector 12 couples light to a TRD temperature probe 14which is located in a rugged high temperature environment showngenerally at 13, such as an aircraft engine. Other uses include thermalprofile characterization. Temperature probe 14 contains luminescentmaterial which luminesces upon excitation by the light carried in fiber10. The luminescent material luminesces most efficiently at a specificmaterial dependent wavelength, and emits luminescent radiation around acharacteristic wavelength as well. Source 4 is selected to provideradiation of such a wavelength as to excite the luminescent material.Source 4 radiates for a preselected time t_(ON) before turning off. Theluminescence is sampled after source 4 has substantially turned off orhas suddenly decreased in intensity. The luminescence decayssubstantially exponentially with time and the exponential time constantof the luminescent emission is responsive to the temperature. Theluminescent intensity is substantially of the form: ##EQU1## where t isthe time and τ is the thermally responsive exponential time constant.

The luminescence is coupled to optical fiber 10 through connector 12 andon to coupler 8. Coupler 8 passes the emissive luminescent radiationover an optical fiber 16 to a detector 18. Alternatively, optical fiber16 may be eliminated. The detector comprises a silicon photodiode havingpeak responsivity substantially in the same band of wavelengths as theluminescence emission. The diode converts the luminescence to a currentrepresentative of the luminescent intensity. A stage of amplification istypically included between the detector and signal processingelectronics, since detector output at 19 typically is approximately afew nanoamperes. Electronics 20 receives a signal representative of theluminescence, samples such signal over two overlapping time intervalsand provides a difference signal as a function of the averages of thesignal on 19 in each of the overlapping time intervals. Next,electronics 20 sends a signal to turn source 4 on, thereby exciting TRDprobe 14 again. In such an iterative fashion, electronics 20 adjusts theoverlapping time intervals to minimize the difference signal andprovides the temperature as a function of one of the parameters in thetime intervals.

In FIG. 2, the electronics of the TRD sensing application shown in FIG.1 are detailed. Control circuit 20 include signal processing circuit 50,comparison circuit 52 and preamp circuit 54. A luminescent means 56comprises source 4, temperature probe 14 and a detector 18. Cabling 22connects control circuit 50 to source 4 and cabling 19 connects detector18 to preamp circuit 54 within control circuit 20.

FIG. 3 includes representations of signals as a function of timecorresponding to signals labelled on FIG. 2. In FIG. 3, the sectionbelow label I shows signal response before steady state is reached andthe section below label II shows steady state signal response. Thevertical scale for signal 108,109 and 110 is expanded. In FIG. 2, source4 light output, shown at 100, has variable pulse height, width andperiod and is described further below. Detector 18 output, shown at 101,is coupled to preamp circuit 54 over cable 19. Preamp circuit 54comprises a preamp 60, which amplifies signal 101 with minimumdistortion and noise to produce signal 102 having an amplitude k₁. Anautozero circuit 62 substantially removes a DC component k₂ from inputsignal 102 to provide signal 103 for comparison circuit 52. Autozeromeans 62 comprises an integrating amplifier and summing amplifier whichdo not substantially distort the AC components of signal 102 as wouldother means for autozeroing such as a capacitor or a high pass filter.

Comparison circuit 52 receives two switch control signals, shown at104,105, from signal processing circuit 50. Such signals controlswitching of signal 103 through switches 70,72 to produce signals106,107. Signal 104 becomes active at A, controlling switch 70 to a lowimpedance state so that signal 103 is effectively connected to a lowpass filter 74 through switch 70. Signal 104 becomes inactive at D,controlling switch 70 to a high impedance state so that signal 103 iseffectively disconnected from filter 74. In like fashion, signal 103 iseffectively connected to low pass filter 76 through switch 72 betweenthe times B and C. Signals 104, 105 are overlapping signals since theactive period for one of the signals is inclusive of the active periodof the other one. The combination of switches 70,72 and control signals104,105 effectively generate two samples of signal 103, one of whichoverlaps the other in time. Signal 106 is substantially the same assignal 103 between times A and D, and signal 107 is substantially thesame as signal 103 between times B and C. Lowpass RC filters 74,76filter signals 106,107 to produce filter signals 108, 109 respectively,which are coupled to a differential amplifier 78. Signal 108 issubstantially expressed by: ##EQU2## and signal 109 is expressed by:##EQU3## The signals 108, 109 are each representative of the averagepower of the luminescence during their respective sampled time period.Differential amplifier 78 output 110 is a slowly varying signalrepresentative of the difference between signals 108, 109.

Signal 108 is fed back to autozero circuit 62, which dynamically adjustsk₂ and consequently the DC level of signal 103, and thereby signals106,107,108,109, until the average value of signal 108 is minimized.This minimization of the average value of signal 108 preventsundesirable discharge or charge of the capacitor included in lowpassfilter 74 when switch 70 is inactive. The discharging and chargingaction directly causes signals 108 and 109 to drift. In summary, signal108 feedback into autozero circuit 62 minimizes drift in signals 108,109by minimizing the discharging and charging of the capacitors in lowpassfilters 74 and 76. Alternatively, signal 109 can be used in place ofsignal 108 as feedback to autozero circuit 62. If filters 74 and 76 didnot allow charging or discharging of capacitors during their inactiveperiods then autozero means 62 may be eliminated.

Sampling of signal 103 occurs when source light output 100 is inactive.Such a sampling arrangement is preferable because no stringentrequirements are placed on the optical system to differentiate betweenexcitation and luminescence radiation.

Signal processing circuit 50 operation is based on values of A,B,C,Dwhich satisfy Equation (4). Signal processing circuit 50 changes thesampling duration of the overlapping signals 104,105 until differencesignal 110 is substantially zero: ##EQU4## Equation (4) may beintegrated and simplified to yield: ##EQU5## Note that Equation (5) isnot affected by the value of substantially constant terms k₁ and k₂. Thetimes A, B and C can be generally expressed by Equations (6-8).

    A=ηD                                                   (6)

    B=A+α(D-A)                                           (7)

    C=A+β(D-A)                                            (8)

Setting η=0 simplifies Equations (5-8) and substitution of simplifiedEquations (6-8) back into Equation (5) and rearranging terms yields:##EQU6## Practical solutions to Equation (9) are dependent on noiselevels and a tradeoff between the temperature probe response time andthe needed accuracy, but are found generally where

    α+β<1                                           (10)

    and

    β≈1-2α                                  (11)

After α, β and η are selected, Equations (4),(5) and (9) are solved forD.

In FIG. 4, signal processing circuit 50 operation, preferablyimplemented in analog electronics, is flow charted. A microprocessor canalso be used. At 300, fixed ratios α, β and η are selected. The ratiosare held constant and are initialized at power-up of the temperaturesensing system. Initial values of variables D and t_(ON) are selected. Apulse signal is sent to source 4 to initiate the excitation radiation,shown at 304. After time t_(ON) has elapsed, a signal is sent to turnoff source 4, shown at 306. After time A has elapsed, enable and disablesignals are sent to switches 70,72, shown at 308. Signals 108,109 areproduced as a result of block 308. An incremental change dD in D iscalculated as a function of signals 108,109, shown at 312 and given by:

    dD=χ.sub.0 (x-y)+χ.sub.1 [(x-y)-(X.sub.prev -y.sub.prev)]+χ.sub.2 Σ(x-y)                    (12)

where the quantity (x-y) is proportional to the difference between theaverage luminescent signal in the overlapping sampling intervals, theconstants χ₀, χ₁ and χ₂ are empirically determined and x_(prev) andy_(prev) are the value of x and y resulting from a previous source 4pulse. A new value for D is assigned at 314. New values of variablesx_(prev), y_(prev) and D_(prev) are assigned at 315.

Source 4 temperature T_(s) is measured at 316 using sensing means 5,which is thermally coupled to source 4 and connected to signalprocessing circuit 50 via cabling 2. The on-time of source 4 is storedin variable t_(ON), which is generated as a function of the sourcetemperature T_(s), shown at 318 and given by:

    t.sub.ON =C.sub.1 Df(T.sub.s)                              (13)

where C₁ is an empirically determined constant, D is defined above,T_(s) is the temperature of source 4 and f(T_(s)) is typically a firstorder linear function of T_(s) such as

    C.sub.2 T.sub.s +C.sub.3                                   (14)

where C₂ and C₃ are constants selected to compensate for lowerefficiency of source 4 at elevated source temperatures. The newlygenerated t_(ON) is used by signal processing circuit 50 the next timethe source enable signal is issued. Additionally, signal processingcircuit 50 may also adjust the intensity of source 4 to compensate fortemperature dependent changes in efficiency of source 4. When x issubstantially equal to y at 310, D relates to temperature via a look-uptable at 311.

In FIG. 5, TRD temperature probe 14 has a fiber optic cable 200 abuttedto a quantity of luminescent powder 202 packed into the end of a fusedsilica tube 204. A glass bond 206,206 preferably of low temperaturesealing glass, bonds silica tube 204 to optical fiber 200. There is nobinder holding the powder particles in the tube end; powder 202 isfirmly sandwiched between the end of tube 204 and the tapered end 205 offiber 200. In other words, the spaces between the particles can be avacuum or can be filled with a gas. Silica tube is a preferred materialsince it is readily available in sizes compatible with optical fibers,is easily fused to the optical fiber and has an expansion coefficientclosely matched to that of the fiber so that the powder remains firmlypacked over a wide temperature range. However, the temperature range candictate that other types of tube material be used. Bond 206,206 ispreferably made of glass because its expansion coefficient is close tothat of silica tube 204. Additionally, silica tube 204 and fiber 200 areself aligning for ease of assembly, since fiber 200 diameter is slightlysmaller than silica tube 204 diameter. Fiber 200 conducts light topowder 202, which emits decaying radiation at a characteristicwavelength upon termination of excitation wavelength, where the decayingradiation is substantially exponential and the time constant thereof isthermally responsive.

The luminescent material is in a powdered form which exhibits increasedabsorption characteristics over the characteristics of a solid.Increased effective absorption reduces the amount of luminescentmaterial needed in each probe, thereby minimizing the sensor elementmass and enhancing the thermal response characteristics of the probe.The powder randomly scatters the excitation and the luminescence,enhancing interchangeability in manufacture. A powder made of particleshaving diameters less than two-third fiber 200 diameter maximizes themeasured luminescent signal levels. The diameter of fiber 200 is thelight-carrying portion of the fiber and is exclusive of the buffer andcladding on the fiber.

Powder 202 is made of one or a combination of the following chromiumdoped materials: chromium doped gadolinium scandium gallium garnet(Cr:GSGG), chromium doped gadolinium gallium garnet (Cr:GGG) andchromium doped beryl (emerald). Emerald is also known as Cr:Be₃ Al₂(SiO₃)₆. These materials have thermally responsive exponential decayconstants within the temperature range of interest and high temperaturequenching limits as shown below. The response is a typical numbermeasured at room temperature and will change considerably overtemperature.

    ______________________________________                                        Cr:GSGG       -0.27 μs/°C.                                                                      .sup.˜ 300° C.                       Cr:GGG        -0.32 μs/°C.                                                                      .sup.˜ 400° C.                       Emerald       -0.10 μs/°C.                                                                      .sup.˜ 500° C.                       ______________________________________                                    

Such materials are excited by radiation substantially between 660-690nm, which is produced by commercially available solid-state lightsources. These solid state light sources are more compact and havehigher output power than do shorter wavelength sources. These chromiumdoped materials have a high quantum efficiency relative to other TRDrelated luminescent materials, so that the ratio of energy inluminescent radiation to the energy in the excitation radiation ishigher than other similar materials. The emission spectrum of thesematerials is approximately between 700-900 nm, corresponding to the peakresponsivity of the silicon photodiode detector. Lastly, these materialsare stable at high temperatures.

Such chromium doped materials also exhibit quantum efficiencysubstantially independent of temperature. Quantum efficiency is thepercentage of electrons which cause a photon to be emitted when theyrelax from an excited energy state to their ground energy state. Aquantum efficiency of 100% indicates that any electron relaxationbetween an excited state and the electron's ground state will emit aphoton. A temperature dependent quantum efficiency occurs when theexcited electron is thermally coupled to at least one additional excitedenergy state, but from which a non-radiative path to the ground stateexists. An increase in temperature yields a stronger coupling to thisnon-radiative state, causing fewer emitted photons and thus a decreasein the quantum efficiency of such material. Consequently, an increase intemperature causes the quantum efficiency of such a material todecrease.

Vanadium doped crystalline halides are alternatively used as particlesin powder 202. Two such vanadium based crystalline halides are V₂ ³⁰:KMgF₃ and V₂ ⁺ : NaCl, which are excited by near infrared radiation,770-780 nm. Such near infrared excitation has a significant advantage,since well-developed and highly reliable LED sources are available inthis range which have even higher radiant output than the 660-690 nmsources.

Fiber 200 has a tapered end 205 embedded in luminescent powder 202. Thetaper effectively increases the numerical aperture of the fiber toincrease the angle of acceptance over that of a perpendicularly cleavedfiber. The angle of acceptance defines a volume of powder which receivesthe excitation radiation. Powdered chromium and vanadium basedluminescent materials are highly effective absorbers so that the volumeof excited powder remains relatively close to the tapered probe tip.Because of the increased acceptance angle and the excited powder volumeremaining close to the probe tip, the amount of luminescence coupledback into fiber 200 also increases, thereby increasing luminescentsignal level over that of a perpendicularly cleaved fiber. A threefoldincrease in luminescent signal level has been measured when the diameterof bulk fiber 200 is 200 μm and the tip is tapered. The angle of thetaper is typically about 12°. In general, the light-carrying core of theend of fiber 200 should have a smaller diameter than that of thelight-carrying core of the bulk of fiber 200.

In practice, the taper of the tip is formed by locally heating a sectionof glass fiber while pulling on each end until the fiber separates intwo sections. The angle of taper depends on the time elapsed during thepulling and the amount of heat applied to the fiber. In some cases, thepulling process results in a tapered fiber that is tilted such that thetip of the taper is not concentric to the fiber center. The end of thetaper is typically rounded when viewed microscopically. Chemical etchingmay also be used to form the tapered tip, and is sometimes used where afine degree of control over the taper is required.

There are various ways to increase the numerical aperture of fiber 200.A tilted tapered fiber with rounded end is shown in FIG. 5A.Alternatively, the fiber end is cleaved and a separate lens is coupledto the fiber, as shown in FIG. 5B. In another embodiment, an integralbead or a ball lens is formed on the end of fiber 200, as shown in FIG.5C.

FIG. 6 shows an alternate temperature probe 300 having blackbodytemperature means and TRD luminescent means. An high temperature opticalfiber 302 is attached to a TRD crystal 306 by a clear glass bond 308. Inhigh temperature sensing applications, glass bond 308 may be omitted inorder to avoid thermal stress. TRD crystal 306 is coated with a thinlayer 310 of inert material such as a metal, effectively creating ablackbody radiator. The material should be at least one penetrationdepth thick, and preferably thicker. The blackbody radiation of theinert material intensifies as does the thickness, since a thickermaterial promotes absorption. A blackbody radiator has spectral emissioncharacteristics varying as a function of temperature according to theStefanBoltzmann Law.

TRD luminescence has a practical high temperature limit, while blackbodyemissions at low temperatures suffer from low signal levels.Consequently, a probe based on both TRD and blackbody temperaturesensing senses a wider range of temperatures than either of the twosensing methods without sacrificing sensitivity. The TRD luminescencemechanism is limited at higher temperatures by quenching, where highertemperatures modify electron energy states such that energy emitted fromelectron relaxation is no longer in the form of photons. Low temperaturelimit of miniature fiber blackbody radiation depends on various factorssuch as fiber size, system losses, the detector and fiber used, but ispractically limited at about 500° C.

Surface finish for crystal 306 affects the amount of luminescence whichexits the crystal into the fiber, altering luminescent signal levels andslightly changing decay times from one crystal to another, therebyaffecting interchangeability. Sandblasting, or other means of uniformlyroughening the surface reduces differences between the signal level fromcrystals. Alternatively, a coating of glass or similar material havingan index of refraction lower than that of the sample allows luminescenceto exit the crystal more efficiently, thereby improving signal levelwhile simultaneously improving interchangeability.

Radiation trapping, where part of the luminescence signal is reflectedback into the crystal, is responsible for the interchangeabilitydiscussed above. It is generally undesirable since it increasesvariations in luminescent signal level and measured decay times betweenvarious crystals. Radiation trapping becomes significant when theabsorption length of the emitted light is substantially the same as orless than the path length of the light within the crystal. A polishedsurface finish reflects more luminescence back into the crystal thandoes an abraded surface. Depending on the spectral absorption andemission characteristics of the luminescence, the trapped radiation hasa finite probability of being absorbed and re-emitted by the luminescentmaterial. The absorption and re-emission is the process responsible forchanging the time rate of decay of the light captured by the fiber.

Electronics for the combination blackbody/TRD sensor include TRDelectronics as in FIG. 2 coupled to blackbody emission sensingelectronics, which sense the total amplitude of broadband light radiatedfrom the blackbody, or employ a wavelength ratioing technique.

In FIG. 7, a system 400 of three LED sources 401,402,404 are coupled toa fiber optics highway 406 and fiber optics highway segments 406a,406b.Sources 401,402,404 have differing distinctive spectral characteristicscentered around λ₁, λ₂ and λ₃. Three port wavelength division couplers408,410,412 have distinctive spectral characteristics as shown at414,416,418, respectively in FIGS. 7A, 7B and 7C. In sketches414,416,418, the dashed line represents the coupler characteristicsbetween the leftmost coupler port connected to the data highway and theport connected to the probe for each of the couplers 408,410,412,respectively. The unbroken line in such sketches represents the couplerCharacteristics between the rightmost port and the leftmost port foreach of couplers 408,410,412 respectively. Coupler 408 diverts a narrowband of frequencies centered around λ₁ from the highway to probe 420,but substantially transmits those wavelengths centered around λ₂ and λ₃across the highway, see 414. In like fashion, coupler 410 exclusivelydiverts those wavelengths centered around λ₂, see 416, and coupler 412exclusively diverts those wavelengths centered around λ₃, see 418.

TRD probes 420,422,424 include luminescent material in powdered form andare connected to couplers 408,410,412 respectively by optical fibers426,428,430. The luminescent material in probes 420,422,424, which canalternately take a crystal form, a powder embedded in a binder, or apowder not imbedded in a binder, is excited by radiation including thosewavelengths λ₁, λ₂ and λ₃. The luminescent material emits luminescencecentered about λ₄, shown at 432,434,436 Couplers 408,410,412 divertwavelengths centered around λ₄.

For example, coupler 408 receives excitation centered about λ1 fromsource 401 over highway 406 and substantially diverts it to probe 420over fiber 426. The radiation from source 401 does not substantiallypass to probes 422,424 since coupler 408 does not pass radiationcentered around λ₁ to highway 406a. Luminescence from probe 420,centered about λ₄, is slightly attenuated by coupler 408 but is passedto electronics 403 over highway 406. Electronics 403 provide an outputrepresentative of the thermally responsive time constant τ and are ofthe form discussed above in FIG. 2. In similar fashion, source 402 sendsexcitation centered about λ₂ over highway 406 to coupler 408, whichsubstantially passes such excitation between the coupler's leftmost portand the rightmost port. Such excitation reaches coupler 410 over 406adiverts the wavelengths centered about λ₂ to the coupler's bottom port,over the fiber 428 to probe 422. Such excitation is substantiallyblocked from reaching probes 420, 424 because couplers 408,412 do notdivert wavelengths centered about λ₂ and coupler 410 effectively divertssuch wavelengths toward probe 422. Luminescence from probe 422, centeredabout λ₄ is slightly attenuated by the return path to electronics 403through coupler 410, highway segment 406a, coupler 408 and highway 406.The number of couplers is practically limited by the luminescent signalstrength after passing through the couplers, which must be on the orderof a few nanowatts in order for electronics 403 to distinguish signalfrom noise. Luminescence from probe 424 is excited by excitation fromsource 404 and passes through couplers 412,410,408 on the path toelectronics 403 in a similar fashion.

In this embodiment, couplers 408,410,412 are three-port wavelengthdivision couplers. Alternatively, a single triple port wavelengthdivision coupler can be used in place of couplers 408,410,412.

The order of the source excitation outputs is arbitrary, as is thelength of the on-time. However, the pulses must be sufficiently long toexcite luminescence and the time between the pulses must be sufficientlylong for the resultant luminescence to reach the electronics. Othermodifications, such as the number of probes, may be made to themultiplexed system, but several requirements must be met. One lightsource is required for each probe which is multiplexed. No two lightsources emit the same wavelength, but all the wavelengths are within theband of wavelengths which excite the luminescent material in each of theprobes.

What is claimed is:
 1. A TRD optical temperature sensor comprising anelongate optical fiber having a luminescent material at one end so as tooptically communicate therewith, the luminescent material providingluminescent optical emissions having a thermally responsivetime-rate-of-decay, the luminescent material comprising a vanadium-dopedhalide.
 2. A TRD optical temperature sensor according to claim 1 whereinthe luminescent material is V₂ ⁺ :KMgF₃ or V₂ ⁺ :NaCl.
 3. A TRD opticaltemperature sensor according to claim wherein the luminescent materialis a vanadium-doped halide which is excitable to luminesce by radiationin the range of 770 to 780 nm.